Various configurations and implementations of electronically reconfigurable diffraction gratings fabricated using MEMS technology are disclosed, for example, in U.S. Pat. No. 5,841,579 by Bloom, et al.; U.S. Pat. No. 5,757,536 by Ricco, et al.; and U.S. Pat. No. 5,999,319 by Castracane. These MEMS-based electronically reconfigurable diffraction gratings offer new and unique degrees of freedom in controlling diffraction of light from a grating, bringing qualitatively new potential to this centuries-old optical device.
The primary advantage provided by these reconfigurable diffraction gratings is the elimination of mechanical tuning and the advent of dynamic control and programmable tuning of the diffraction pattern. The practical applications of this device, however, have been limited so far. This device has found applications mainly in spectroscopy and digital display applications. As will be disclosed herein, a novel and nonobvious application of the electronically reconfigurable diffraction grating is to an improved optical pickup device that can be implemented in CD players, DVD players, computer storage devices, and laser based surface profilometers.
A typical layout of an existing optical pickup device, for example, in a CD player, is shown in FIG. 1. A solid state laser diode 102, typically emitting in the near IR, emits optical power in a wedge shaped beam with a typical divergence of 10×30 degrees in the X and Y directions, respectively. A diffraction grating 104 splits the output laser beam into a main (zero order) beam 106 and two (1st order) side beams designated as first 1st order side beam 108 and second 1st order side beam 110. In these existing prior art devices, only the zero and first order beams (106, 108 and 110 respectively) are used. The higher order beams (second order and above) are not used. The zero order beams are used to read content information, e.g., music, video, computer data, etc., from the disk. The 1st order side beams 108 and 110 are used for tracking the track on the disk which is being read (tracking information). The tracking servo mechanism in a typical CD player or other device that would use an optical pickup, maintains the 1st order side beams 108 and 110 by keeping the amplitude of the reflection of these two 1st order side beams 108 and 110 equalized, as measured by the system's photodetector in a feedback loop arrangement.
Next, the laser beam passes through a polarizer 111, polarizing beam splitter 112, a turning mirror 118, a collimating lens 114, a quarter wave plate 116, and the objective lens 120 before reaching the optical storage media disk 122 (compact, digital video, etc.). The collimated laser beams (the main zero order beam 106 and the two 1st order side beams 108 and 110) pass through the objective lens 120 and are focused to diffraction-limited spots on the information layer of the disk, known as the pits. The reflected beam retraces the original path up until it passes through the polarizing beam splitter 112 at which point it is diverted toward the photodetector array 124. Additional focusing optics 126 are used to focus the reflected main zero order beam 106 on the quadrant photodetector 128 and the 1st order side beams on individual photodetectors 130, located on the side of the quadrant detector 128 in the photodetector array 124, as shown in FIG. 3.
For reference, FIG. 2 shows a typical recorded fragment on a CD or alternative optical storage media. Shown in FIG. 2 are the pits 232 and the coast 234. The pits 232 comprise the information content storage layer of the disk and are where the main zero order beam 106 is focused to. Assuming the optical storage media 450 is round, a pit line 233 would contain all pits 232 located at the same radius and is represented in FIG. 2 as the dotted line through the center of laterally adjacent pits. Similarly, the coast 234 is defined as the area between adjacent pits 232 both laterally and longitudinally. The coast line 235 is defined as the median line equidistant between successive pit lines 233. The typical width of the pits 232 is 0.5 micron (shown as the vertical distance 231 across the pit) and the pitch is 1.5 micron (defined as the distance between pits lines 233 and shown as the vertical distance 237) which makes the width of the coast 234 1 micron (shown as vertical distance 239). This is where the 1st order side beams 108 and 110 are focused to.
As mentioned above, the photodetector array 124 typically comprises a quadrant photodetector 128 (labeled A, B, C, D) and two individual photodetectors 130 (labeled E and F) that are located on the wide extremes of the quadrant detector 128. For reference, this photodetector array configuration is shown in FIG. 3 along with the typical reflected beams. The individual photodetectors 130 located on the wide extremes are typically used to detect and measure the reflected 1st order side beams 108 and 110, while the quadrant detector 128 is used to measure the reflected zero order beam 106.
The two reflected 1st order beams 108 and 110 are used for horizontal tracking. When the focal spot shifts sideways from the center of the pits 232, one of the side spots starts leaving the coast 234 and covering some of the pit 232 area creating an obvious change in reflected intensity. The resulting difference in the signals from the two individual photodetectors 130 is used as an error signal in the feedback loop for horizontal tracking. The width of the coast 234 is typically twice the width of the pits 232, as shown in FIG. 2, to provide for the differential feedback signal.
The reflected zero order beam 106 is used in a feedback configuration to establish focus on the pit 232 of the optical storage media 450 for information content readout. When in focus, the reflected zero order beam 106 is circular on the quadrant photodetector 128 as shown in FIG. 3. When out of focus in one direction, the reflected zero order 106 is diagonally elliptical across quadrants B and C as shown in FIG. 3a, while being out of focus in the opposite direction produces an ellipse across quadrants A and D, as shown in FIG. 3b. The focus is maintained by sampling the intensities of the diagonal quadrants and comparing. In other words, the sum of the intensities of quadrants B and C is compared to the sum of the intensities of quadrants A and D in a feedback loop in order to maintain focus.
This method of signal detection and processing limits the technology to non-overlapping diffraction limited reflections on the photodetector 128. Any overlap in the zero and +/−first order reflected beams (106, 108, 110), would skew the signals to the photodetector 128, and thereby falsify the horizontal tracking and information focusing feedback. Therefore, the prior art technology is limited in optical storage density to a configuration that provides diffraction limited nonoverlapping signals to the photodetector 128.
The prior art technology, as described above, is further limited to using only the diffracted energy in the zero and first orders, in addition to being limited to nonoverlapping reflections on the readout device. This technology is described, for example, in U.S. Pat. Nos. 5,717,674; 5,475,670; 5,412,631; 5,231,620, 5,128,914, and 5,094,732. These patents teach multiple ways of implementing optical pickup devices utilizing three signals, one from the zero diffractive order and one from each the +/−first orders. In order to increase both the optical storage density and optical readout speed, improvements in the method of signal processing must be realized to overcome these existing limitations.